How can I change the FOV in game? Will that influence my leaderboard score or achievements?
Type this in the console:
plr_fFOVOverride=120
(or whatever FOV you like)
Changing the FOV is not considered a cheat and will not influence the leaderboards or achievements.

My game has crashed, it shows an "application has malfunctioned" message. What should I do?
Please locate these two files: Sam3.log and Sam3.rpt. They should be in the folder "<steam installation>/SteamApps/common/serious sam 3/Log/". Then create a post in this forum and paste the contents of those two files. (They are just text files and you can open them with Notepad.) Please create a new thread for a crash so it's easier to track each user's individual problem.

The game won't even start, or crashes on startup, or exits while starting with error messages about missing files. Why is that? What can I do?
This could be caused by a myriad of reasons, but here are the solutions that help most often:

Always try Verify Game Cache first. This will solve the problem in 99% of such cases.

Make sure the exe file has a correct verifiable signature from Valve Corporation. To do this, select the Sam3.exe file in Explorer, then: Right click, Properties, Digital Signatures tab. You should see a signature in the name of "Valve Corporation". Selecting that and clicking Details, should show a dialog with this message (among other things): "The digital signature is OK.".
If it shows "The certificate in the signature cannot be verified." (Note the exact wording - it didn't say it is wrong, it said it could not be verified!) it means that your OS is missing a common root certificate. Probably due to disabled Windows updates or similar. You can fix it by finding out which certificate is missing and installing it.

Check that the problem is not caused by some firewall or antivirus software. For more information, see next question.

Game crashes on startup for no apparent reason, or behaves weirdly during the game (controls misbehaving, random erratic movements, etc.)
Issues like these with various games have often been reported to be caused by some intrusive antivirus and personal firewall programs. Sometimes just disabling the program in question or excluding Serious Sam 3 from checking may help.

However, with some of those programs, the only remedy is to completely uninstall the program. This is because, even when "disabled", the program is still installed as a low-level system driver and interferes into the process space of the game.

Some such programs that were known to cause problems like this in the past are:

Some versions of ZoneAlarm (Caused by ISWSVC.exe which is activated by ZoneAlarm Toolbar or ZoneAlarm Browser Security)

Some versions of Avira (It is not certain whether it requires complete uninstall of Avira)

Some versions of NOD32 (Sometimes requires complete uninstall of NOD32)

Some of the sounds in the game are missing or do not play. What should i do?
Check your Windows sound settings and make sure that the speaker configuration is correct.

What do I need to run the dedicated server for Serious Sam 3?
Go to the Library>Tools section of the Steam client and find the item named "Serious Sam 3 Dedicated Server" and install it. Then just start the tool, if you want to run on default settings. For information on customization, please see Help/DedicatedServer_Readme.txt .
The server can also be installed using the HLDSUpdateTool, using gamename "serioussam3".